Wireless communication system using surface acoustic wave (SAW) second harmonic techniques

ABSTRACT

A system is for encoding information on a passive surface acoustic wave (SAW) device. The system includes a requesting unit configured to wirelessly transmit an impulse signal. The impulse signal is a signal that includes only a single pulse. A SAW device has an interdigital transducer (IDT) configured to physically store coded data. The SAW device is configured to receive the impulse signal. In response to receiving the impulse signal, the SAW device excites the IDT to generate a coded signal that includes the stored coded data and frequency components at a fundamental frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 13/325,262, filedDec. 14, 2011, which is a continuation of U.S. application Ser. No.12/778,818, filed May 12, 2010 (now U.S. Pat. No. 8,099,048), which is acontinuation of U.S. application Ser. No. 11/863,330, filed Sep. 28,2007 (now U.S. Pat. No. 7,747,220), which is a continuation of U.S.application Ser. No. 10/946,420, filed Sep. 21, 2004 (now U.S. Pat. No.7,292,822), which is a continuation of U.S. application Ser. No.09/863,944, filed May 23, 2001 (now U.S. Pat. No. 6,825,794), whichclaims priority from U.S. Provisional Application No. 60/209,152, filedJun. 2, 2000, all the above applications hereby incorporated byreference.

TECHNICAL FIELD

This relates to short range communications using surface acoustic wave(SAW) expanders and compressors.

BACKGROUND

SAW technology is well known for its excellent radio frequency (RF)performance, low cost and small size. SAW is a passive thin filmtechnology that does not require any bias current in order to function.SAW expanders and compressors have been used in RADAR applications formany years.

The basic “building block” of SAW expanders and compressors is theinterdigital transducer (IDT) such as shown in FIG. 1. An IDT 10 is aseries of thin metal strips or “fingers” 12 fabricated on a suitablepiezoelectric substrate 14. One set of fingers is connected to aninput/output terminal 16, while the opposite set of fingers is connectedto another terminal 18. In single-ended IDTs, terminal 18 is grounded.For differential input signals however, terminal 18 is a pulseinput/output terminal. Spacing “W” between IDT segments is adjusted toconform to the desired chip period of the coded sequence. When excitedby a narrow pulse at terminal 16, the IDT generates a coded output SAWwhich propagates in both directions perpendicular to the fingers 12. Ifa similarly coded SAW impinges on the fingers 12, then anautocorrelation function is performed and a peak, with associated sidelobes, is generated at terminal 16. These abilities of SAW expanders andcompressors are well known in the prior art, having been demonstratedfor example in Edmonson, Campbell and Yuen, “Study of SAW PulseCompression using 5×5 Barker Codes with Quadraphase IDT Geometries”,1988 Ultrasonics Symposium Proceedings, Vol. 1, 2-5 Oct. 1988, pp.219-222.

Thus the structure shown in FIG. 1 can operate as both a SAW expander,generating a SAW output from a single pulse input, and a SAW compressor,generating a single pulse or peak output from a SAW input. Terminal 16,as well as terminal 18 in differential IDTs, is both a pulse inputterminal and a pulse output terminal. Conversion of an output SAW intoan electrical signal for further processing in conventionalcommunications circuits and subsequent transmission through an antennais accomplished by adding a transmit IDT 24, aligned with the IDT 22, asshown in FIG. 2. Both IDTs can be fabricated on the same substrate 14. ASAW output from IDT 22 is converted into an electrical signal by TX IDT24. A SAW receiver would have the same structure as in FIG. 2. A signalinput to a receive IDT from receiver processing circuitry would beconverted to a SAW which is input to IDT 22. Like the IDT 22, the TX IDT24 may be a differential IDT, wherein the grounded lower terminal wouldbe a pulse output terminal.

The geometry of adjacent IDT fingers 12 is shown in FIG. 3, where Tf isthe width of a metallized finger 12 and Ts is the width of the spacebetween the fingers 12. In typical designs both Tf and Ts are equal to aquarter of a wavelength, λ/4. Since wavelength is inversely proportionalto frequency of operation, higher frequency IDTs require thinner fingers12 located in close proximity to each other, which complicatesfabrication and reduces fabrication yields. For example, for a typicalSAW system operating in the Industrial, Scientific and Medical (ISM)band at 2.4 GHz the λ/4 dimension could be in the order of 0.425microns, depending upon the substrate chosen.

Previous communications system designs sought to overcome thesemanufacturing difficulties by using lower frequency SAW expanders andcompressors having larger and further spaced fingers in conjunction withmixers and local oscillators, as shown in FIG. 4. In the typical priorart communication system 30, the lower frequency 266 MHz signalgenerated by transmit IDT 20 is up-converted in mixer 34, which receivesa 734 MHz signal from local oscillator 36. The output from mixer 34 isfiltered in high pass filter 38 to produce a 1 GHz signal which istransmitted through antenna 40. On the receive side, the process isreversed in antenna 42, mixer 44, low pass filter 46 and receivecompressor IDT 20′. As discussed above, transmit IDT 20 and receive IDT20′ have similar structure. Undesirably, the mixers 34 and 44,oscillator 36 and filters 38 and 46 from the communications system 30,result in additional cost, power consumption, occupation in space and amuch complex system than is desired for low-cost, low power, short rangecommunication systems. Therefore, there remains a need in the art toreduce the number of components in such a communication system.

High-frequency communication techniques involving more conventionalnon-SAW based circuits and systems also exist. Bluetooth™ wirelesstechnology is one such prior art example. Bluetooth is a de factostandard, as well as a specification for small-form factor, low-cost,short range radio links between mobile PCs, mobile phones and otherportable wireless devices. The current Bluetooth short rangecommunications specification operates in the 2.4 GHz (ISM) band;however, in reality the technology for mobile communication devicesinvolves undesirable high cost, substantial power consumption andrelatively complex hardware.

SUMMARY

It is an object of the present invention to overcome some of thedrawbacks of the prior art.

It is also an object of the present invention to provide a low costSAW-based communication method and system. As an illustrative example ofthe cost reduction, SAW devices utilized by the present invention forfiltering at near-ISM band frequencies may cost approximately $1.00each. In contrast, a comparable semiconductor Bluetooth solution maycost greater than $10.00.

It is a further object of the invention to provide SAW-based transmitand receive units which are easily manufactured. The manufacturingrequired for the present invention allows for SAW fabrication thatutilizes simple, single layer photolithographic techniques.

Another object of the invention is to provide a low power SAW solutionfor short range communications. The SAW uses passive thin filmtechnology and requires only a pulse to excite and produce an RFwaveform. Likewise it can perform an autocorrelation function passively.This compares to prior SAW techniques which require frequency conversioncircuitry such as mixers, filters and oscillators, and the complexBluetooth techniques that require separate receive, transmit andprocessing circuitry. In mobile communication environments, powerconsumption and size are of primary importance.

A still further object of the invention is to provide a SAW-basedcommunication arrangement which occupies minimal space. A complete SAWpackage in accordance with the invention is in the order of 3 mm×3 mm.

The inventive SAW system reduces manufacturing complexity and cost andincreases production yields by exploiting second harmonic componentsproduced by expander/compressor IDTs. This allows the IDTs to befabricated with larger finger widths than would be required according toknown IDT methods and devices.

In the invention, a wireless communication system comprises anexpander/compressor interdigital transducer (IDT) which produces asurface acoustic wave (SAW) output comprising frequency components at afundamental frequency and a plurality of harmonic frequencies whenexcited with an electric input signal and produces an electric outputsignal when excited by a SAW input at the fundamental frequency or oneor more of the plurality of harmonic frequencies, a transmit IDTpositioned adjacent to the expander/compressor IDT and switchablyconnected to an antenna, and a receive IDT positioned adjacent to theexpander/compressor IDT and switchably connected to the antenna, whereinthe transmit IDT and the receive IDT are configured to operate at one ofthe harmonic frequencies.

In accordance with another aspect of the invention, a communicationsystem comprises an expander IDT configured to produce a SAW outputhaving a fundamental frequency and a plurality of harmonic frequencieswhen excited with an electric input signal, a transmit IDT positionedadjacent to the expander IDT and connected to an antenna, a receive IDTconnected to the antenna; and a compressor IDT positioned adjacent tothe receive IDT and configured to produce an electric output signal whenexcited by a SAW input comprising the fundamental frequency or one ormore the plurality of harmonic frequencies, wherein the transmit IDT andthe receive IDT are configured to operate at one of the harmonicfrequencies.

The transmit IDT converts a SAW into an electric signal for transmissionvia the antenna and the receive IDT converts an electric signal receivedvia the antenna into a SAW. The fundamental frequency may be 1.2 GHz andthe transmit IDT and receive IDT operate at the second harmonicfrequency of 2.4 GHz. Wireless communication systems according to theinvention may be installed in both a wireless mobile communicationdevice and a wireless earpiece detachable therefrom, to provide forcommunication between the mobile device and the earpiece. In a furtherembodiment of the invention, a SAW-based wireless communication systemis installed in a wireless mobile communication device, a wirelessearpiece detachable therefrom and a holder for the mobile deviceconnected to a personal computer (PC), to provide for communicationbetween the device and the PC through the holder, the device and theearpiece, and the earpiece and the PC through the holder.

The electric input and output signals associated with any of the IDTsmay be either unbalanced or differential signals.

An expander/compressor IDT, or an expander IDT and a compressor IDT arepreferably configured to embody a code and thereby produce a coded SAWoutput when excited with an input electric pulse and an output electricpulse when excited by a coded SAW input. The code embodied by these IDTsmay be a Barker code such as a 5-bit Barker code, and may be used forexample to represent identification information for an article withwhich the wireless communication system is associated.

According to a further aspect of the invention, a passive wirelesscommunication system comprises an antenna for receiving communicationsignals and converting the received communication signals into electricantenna output signals and converting electric antenna input signalsinto output communication signals and transmitting the outputcommunication signals, a first IDT connected to the antenna andconfigured to produce first SAW outputs in response to the electricantenna output signals and to produce the antenna input signals inresponse to first SAW inputs, a second IDT positioned adjacent to thefirst IDT and configured to produce a second SAW output comprisingfrequency components at a fundamental frequency and a plurality ofharmonic frequencies when excited with an electric signal and to producean electric signal output when excited by a second SAW input at thefundamental frequency or one or more of the plurality of harmonicfrequencies, and a termination circuit connected to the second IDT,wherein the first IDT is configured to operate at one of the harmonicfrequencies, the termination circuit causes the second IDT to reflect asecond SAW output toward the first IDT in response to each first SAWoutput produced by the first IDT, and the first IDT produces an antennainput signal in response to each reflected second SAW output from thesecond IDT. The second IDT in such a passive system may be configured toembody a code.

In a particular embodiment of this aspect of the invention, a passivewireless communication system further comprises a third IDT which ispositioned between the first IDT and the second IDT and reflects a thirdSAW output toward the first IDT in response to the first SAW outputproduced by the first IDT, wherein the first IDT produces a secondantenna input signal in response to the reflected second SAW output fromthe third IDT. The passive wireless communication system may alsoinclude a fourth IDT which is positioned adjacent to the second IDT on aside of the second IDT opposite to the third IDT and reflects a thirdSAW output toward the first IDT in response to the first SAW outputproduced by the first IDT, wherein the first IDT produces a thirdantenna input signal in response to the reflected third SAW output fromthe fourth IDT. When the passive wireless communication system includesa third IDT and/or fourth IDT, these IDTs are preferably configured tooperate at one of the harmonic frequencies, which may be the same as theharmonic frequency at which the first IDT operates.

The third and fourth IDTs, like the second IDT, may be connected to arespective termination circuit. A termination circuit is preferablyeither an open circuit, a short circuit or a sensor circuit. The passivewireless system preferably receives communication signals from a remoteinterrogation system, and through operation of the IDTs and terminationcircuit, automatically and passively responds to the remoteinterrogation system.

An IDT which may be used in embodiments of the invention preferablycomprises a pair of substantially parallel electrically conductive railsand one or more groups of interdigital elements, each group comprising aplurality of interdigital elements. Each interdigital element isconnected to one of the rails and extends substantially perpendicularthereto toward the other rail. Any such IDT may be configured to embodya particular code as determined by a connection pattern of theinterdigital elements in each group. A coded IDT performs a passiveautocorrelation function on a SAW input based on the code to therebyproduce an electric pulse output when the SAW input is similarly coded.

A SAW system according to the invention may be employed in the design ofvirtually any new short range wireless communication system, for exampleto enable communication between an earpiece unit and an associatedmobile communications device, as described briefly above and in furtherdetail below. The inventive systems may also replace RF signalgeneration circuitry in existing short range communications system,including for example “Bluetooth” systems. A further system inaccordance with the invention may be employed in “smart” identificationtag systems and remote interrogation systems such as inventory systemsand meter reading/telemetry systems.

Further features of the invention will be described or will becomeapparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, preferredembodiments thereof will now be described in detail by way of example,with reference to the accompanying drawings, in which:

FIG. 1 shows an IDT;

FIG. 2 is a block diagram of a conventional SAW-based transmit IDT;

FIG. 3 is an illustration of typical finger geometry in an IDT;

FIG. 4 is a block diagram of a prior art SAW-based communication system;

FIG. 5 is an IDT adapted for second harmonic operation;

FIG. 6 shows a representation of a first embodiment of the invention;

FIG. 7 is a differential implementation of the first embodiment;

FIG. 8 illustrates an autocorrelation function of a 5 bit Barker code;

FIG. 9 is a second embodiment of the invention;

FIG. 10 is a differential implementation of the second embodiment;

FIG. 11 represents a system in which the invention could be implemented;

FIG. 12 illustrates a third embodiment of the invention;

FIG. 13 is a differential implementation of the third embodiment;

FIG. 14 shows a variation of the third embodiment;

FIG. 15 is a differential implementation of the system of FIG. 14; and

FIG. 16 is a block diagram of a system in which the third embodimentcould be employed.

DETAILED DESCRIPTION

As discussed above, the lithographic process to produce SAW devices athigher frequencies is difficult due to the very small finger width. At2.4 GHz, the wavelength would be approximately 1.7 microns, requiring afinger width of 0.425 microns depending on the substrate chosen. Thisvery small width will affect the overall yield of the fabricationprocess and will impact on the price of the devices.

A solution to this problem would be to fabricate the device to operateat 1.2 GHz to produce a wavelength of 3.4 microns using unique fingergeometry and then take advantage of the second harmonic that the devicewill support. This will allow for a more relaxed lithographic processand increase production yield, as the lines are not as thin and arespaced farther from each other.

The use of second harmonic IDT geometries has been well know for severalyears, see for example CAMPBELL and EDMONSON, “Conductance Measurementson a Leaky SAW Harmonic One-Port Resonator”, IEEE Transactions onUltrasonics, Ferroelectrics and Frequency Control, Vol. 47, No. 1,January 2000, pp. 111-116, but has never been applied to expanders orcompressors. FIG. 5 illustrates an example of the finger configurationfor a second harmonic SAW device with 2 chips (+ and −). In FIG. 5 andsubsequent drawings, the substrate 14 has been omitted for clarity, butit is to be understood that IDT structures may be fabricated on a commonsubstrate.

As shown in FIG. 5, a so-called “three-finger” IDT, in which each of thefour groups of fingers includes three fingers, is required for secondharmonic operation. Corresponding fingers of each group are separated bya distance ‘a’ equal to fundamental wavelength λ₀. Each finger and spacein this three-finger IDT therefore has a width ‘b’ of λ₀/6.

The two double fingers in each group start out at the left hand side ofthe IDT attached to the top rail, but beyond the centre line they areattached to the bottom rail. This indicates a 180° phase shift as whatis derived from a + and − configuration. As stated above, the finger andspace width of the second harmonic IDT is λ₀/6. For a 2.4 GHz secondharmonic output, the fundamental frequency is 1.2 GHz, corresponding toa wavelength λ₀=3.4 microns. The required finger width will beλ₀/6=0.567 microns instead of the 0.425 microns finger width for a 2.4GHz IDT. FIG. 5 shows a single-ended IDT with a grounded lower terminal,but a differential IDT design could also be employed.

According to a first preferred embodiment of the invention, with an IDTarrangement which can directly produce a high frequency output signal, aSAW-based communications system could comprise an expander/compressorIDT 52, a transmit (TX) IDT 56 and a receive (RX) IDT 60. Thesestructures are in-line with each other as shown in FIG. 6. As discussedabove in relation to FIG. 2, these structures may be placed on asuitable piezoelectric substrate using thin film lithographicprocedures.

A narrow pulse which represents digital data and can be generated byusing simple digital circuitry or an existing data source is injectedinto the middle IDT 52 of FIG. 6 through pulse input and output terminal54 to activate a piezoelectric effect that converts electrical tomechanical (acoustic wave) motion. The acoustic waves can be codeddepending on the geometry of the IDT 52. These acoustic waves thenpropagate within the substrate to the TX IDT 56. The coded acousticwaves are then transformed to an electrical coded RF signal within theproximity of the TX IDT 56. When the TX IDT 56 is attached to a suitableantenna 58 through the switch 62 and band pass filter 57, the coded RFsignal can propagate throughout the air.

The same device can then perform in a similar reciprocal fashion. Acoded electrical signal that enters the RX IDT 60 via the antenna 58,band pass filter 57 and switch 62 generates an acoustic wave thatpropagates towards the middle expander/compressor IDT 52. Anautocorrelation function is passively performed in the IDT 52 and if thecoded waveform from the RX IDT 60 matches with the code on theexpander/compressor IDT 52, a peak is generated at the pulse input andoutput terminal 54.

As discussed above, any of the IDTs shown in FIG. 6 could be implementedas differential IDTs. A fully differential system is shown in FIG. 7. Incomparison with the system of FIG. 6, all of the grounded terminals inFIG. 7 are pulse input and output terminals in FIG. 7. Although twoswitches 58 and 58′ are shown, a single differential switchingarrangement may be used. As indicated by the multiple connections inFIG. 7, the filter 61′ and antenna 62′ must also be differentialcomponents. Expander/compressor IDT 52′ may be single-ended, withterminal 55′ grounded as shown in FIG. 6, or differential, whereinterminal 55′ is a pulse input and output terminal. The differentialsystem in FIG. 7 operates similarly to the system of FIG. 6, as will beapparent to those skilled in the art.

The peak produced by an expander/compressor IDT such as 52 or 52′ canrepresent digital data. For example, in accordance with an on-off keyingtechnique, following an intialization or synchronization sequence, thepresence of a peak within a bit period may be interpreted as a ‘1’ databit, whereas the absence of a peak would represent a ‘0’ bit.

The coding of the expander/compressor IDTs 52, 52′ and the associatedautocorrelation function performed by the IDTs as discussed above aredetermined by the finger geometry of the IDT. A preferred IDT codingscheme is a Barker code. Barker codes are particularly useful for IDTcoding, since they minimize the energy in the side lobes associated witha compressed pulse generated by the autocorrelation function performedon a SAW input to an expander/compressor IDT. In FIG. 6 for example, theexpander/compressor IDT 52 embodies a 5 bit + + + − + Barker code.

FIG. 8 shows an example of the autocorrelation function performed by theexpander/compressor IDT 52 of FIG. 6 when a signal received through theantenna 58 and switch 62 is converted to a SAW by RX IDT 60. Theautocorrelation function is mathematically equivalent to a series ofshift and add operations as shown in FIG. 8 and generates the peak andassociated side lobes shown at the bottom of FIG. 8. The amplitude ofthe autocorrelation peak is proportional to the code length N, which is5 in the example shown in FIG. 8, whereas the side lobes are amplitude1. This passive autocorrelation decodes received signals that weregenerated with an identically-coded IDT.

In the system of FIGS. 6 and 7, only the expander/compressor IDTs 52,52′ must be coded. As discussed above, Barker codes are preferred. Sincethe amplitude of the autocorrelation peak generated when a receivedsignal is compressed by a Barker-coded expander/compressor IDT isdependent on the length N of the Barker code, higher-length codes aremost preferred. For example, the maximum length known Barker code withN=13 (+ + + + + − − + + − + − +) will generate an autocorrelationwaveform similar to that shown in FIG. 7, but having a peak of amplitude13 and additional side lobes with amplitude 1.

Also evident from FIGS. 6 and 7 are the relative lengths of the RX IDTs60, 60′, the expander/compressor IDTs 52, 52′ and TX IDTs 56, 56′. Byfar the longest IDTs, expander/compressor IDTs 52, 52′, are fabricatedwith a finger width of 0.567 microns to facilitate second harmonicoperation at 2.4 GHz. Only the shorter IDTs 56, 56′, 60 and 60′ must befabricated for 2.4 GHz operation with the smaller finger width of 0.425microns. Therefore, the more stringent manufacturing requirements applyonly to the shorter elements, which will increase production yields.Fabrication of the shorter elements with thinner fingers is considerablyless difficult than fabrication of the much longer expander/compressorIDT with the same finger width. Furthermore, the representations shownin the drawings are simplified views of expander/compressor IDTs. Inreality, the IDTs 52, 52′ will often comprise more than the single setof fingers shown in FIGS. 6 and 7 per code bit.

The antenna switch 62 in FIG. 6 and switches 62 and 62′ in FIG. 7 arerequired to prevent feedback of a transmission signal from the TX IDTs56 and 56′ to the RX IDTs 60, 60′, which would occur if both the TX andRX IDTs were connected to the antennas 58, 58′. Such feedback wouldcause the RX IDTs 60, 60′ to convert the fed back signal to a SAW, whichin turn would propagate through IDT 52, 52′ and cause interference.Switches 62, 62′ similarly prevent a received signal from feeding backthrough the TX IDT 56, 56′. However, small-scale switches of the typenormally employed in such arrangements tend to be prone to failure. Theswitch and associated complex control circuits also occupy space andconsume power. Such problems are critical concerns in highly integrateddevice designs and mobile communications equipment in which SAW systemsaccording to the instant invention could be employed.

A second embodiment of the invention as shown in FIG. 9 eliminates theantenna switches and the problems associated therewith. According to thesecond embodiment, the SAW-based communication system 70 has an expanderIDT 52 a and a compressor IDT 52 b. A pulse representing data input atterminal 54 is converted to a coded SAW by expander IDT 52 a. TransmitIDT 56 then converts the resultant coded SAW into an electrical signalfor transmission via band pass filter 57 and antenna 58. Feedback of thetransmit signal to the RX IDT 60 does not interfere with the IDT 52 a inthe transmit module 80 a. Pulse output 54 b is not read during signaltransmission to prevent erroneous data detection. A signal received atantenna 58 is filtered by band pass filter 57, input to RX IDT 60,converted to a SAW and decoded by autocorrelation in compressor IDT 52 bprovided the received signal code corresponds to the coding of IDT 52 b.The autocorrelation peak is output at terminal 54 b. Although thereceived signal is split between the TX IDT 56 and the RX IDT 60, theSAW generated at TX IDT 56 causes no interference with the receivemodule 80 b. Any pulse output on terminal 54 a during a receiveoperation is ignored.

The IDTs shown in FIG. 9 are similar in construction to those in FIG. 6.The expander IDT 52 a and compressor IDT 52 b are fabricated for secondharmonic operation at 2.4 GHz and coded in the same way as IDT 52 ofFIG. 6. TX IDT 56 and RX IDT 60 operate at a fundamental frequency of2.4 GHz.

Any of the IDTs in FIG. 9 may be differential IDTs, as shown in FIG. 10.In the differential arrangement, terminals of the TX IDT 56 and RX IDT60 shown as grounded in FIG. 9 are pulse input or output terminals inFIG. 10. Although single-ended IDTs are preferred for the expander IDT52 a′ and compressor IDT 52 b′, these IDTs may also be differentialIDTs, in which case terminals 55 a′ and 55 b′ are connected as pulseinput and output terminals, respectively, instead of to ground.

Although the problems associated with the antenna switches 62 and 62′ ofthe first embodiment are eliminated in the second embodiment, transmitand receive signal splitting at the antenna result in signal powerlosses within the system. Any choice between the first and secondembodiments trades off the relatively higher failure rates, controlcircuit complexity, size and power consumption of the first embodimentagainst the signal power losses of the second embodiment.

The arrangements disclosed above can reduce the cost, power consumption,size and complexity of virtually any short range communications system.This SAW based technology will allow communication devices to be placedin power sensitive applications such as a wireless earpiece to allow theuser a longer “talk-time” over Bluetooth devices.

This invention may be incorporated into any situation for whichBluetooth was designed. An illustrative example of a system into which asystem in accordance with the first or second embodiment could beincorporated is shown in FIG. 11.

One contemplated application of the invention is illustrated in FIG. 11,wherein 102 denotes an earpiece, 104 is a mobile wireless communicationdevice and 106 is a holder or cradle for holding the device 104 andcoupling device 104 to a personal computer (PC) 110. In system 100, theearpiece 102, device 104 and cradle 106 incorporate a SAW communicationdevice as disclosed above. This allows a user to communicate audiblybetween the wireless communication device 104, which may for example becarried on their belt or person, and the wireless earpiece 102 with abuilt-in microphone, as indicated at 108 a in FIG. 11. This system couldbe then expanded to include communication between the earpiece 102 andthe personal computer 110, as indicated by 108 b, when a SAW system incradle 106 is attached to the PC via a bus connection. This system maythen be further expanded to include network communications (designated108 c in FIG. 11) between the wireless device 104 on the belt or personwith the PC 110 to incorporate connectivity via small pico-cellnetworks. A further extension of the communication systems according tothe first and second embodiments could be a personal area network (PAN)based on SAW technology rather than the more excessive Bluetoothstrategy.

In a third embodiment of the invention, the second harmonic designtechniques discussed above are applied to passive SAW RF systems. Insuch systems, SAW devices usually perform only as RF expanders. As shownin FIG. 12, such a passive system 120 may comprise two IDTs 122 and 124.IDT 124 is fabricated according to fundamental frequency criteria,whereas IDT 122 operates at a harmonic of the fundamental, as discussedabove. A pulse that has been sent out by a local requesting unit isreceived at the antenna 128 and excites IDT 122 to produce an acousticwave. This wave then propagates to a coded IDT 124 that has a suitabletermination 126 connected across its terminals 132 and 134 to produce areflection coefficient of magnitude 1. Termination 126 could be an openor short circuit termination, which will re-excite the coded IDT 124 toproduce a coded acoustic wave back to the IDT 122 that is connected tothe antenna 128. The result is that an impulse sent out by a localrequesting unit excites a coded IDT which then returns back to therequesting unit a coded RF waveform.

At the requesting unit, autocorrelation of the coded waveform returnedfrom the device 120 would preferably be performed by a DSP or otherconventional signal processing circuitry, such that different codes canbe used for different IDTs such as IDT 124. In order for the requestingunit to passively perform the autocorrelation, a separate coded IDT mustbe provided in the requesting unit for each different code embodied inall devices 120 with which communication is desired. This would severelylimit the number of devices 120 that could be deployed.

The size of the complete SAW device 120, as discussed above, could be onthe order of 3 mm square. This would allow the device to be incorporatedinto labels such as shipping or address labels, equipment name plates,adhesive stickers such as vehicle license plate stickers and other formsof identification tags. The code embodied in the IDT 124 could forexample be a code that provides information about an item to which thedevice 120 is attached. Device 120 could therefore be implemented in anidentification or location system for example.

Although IDT 122 in FIG. 12 is a single-ended IDT, a differential designis also contemplated, as shown in FIG. 13.

In FIGS. 12 and 13, IDT 124 is shown as a coded IDT that produces acoded reflected SAW that can provide information to the requesting unit.However, in the systems of FIGS. 14 and 15, the IDTs are not coded. Asshown in FIG. 14 for example, the passive communication system includesfour IDTs, 122, 136, 138 and 142, of which IDTs 136, 138 and 142 arefabricated as fundamental frequency components. IDT 122 is fabricatedfor operation at a harmonic frequency of fundamental. The terminals ofIDT 136 are either open circuited as in FIG. 14 or short circuited suchthat a SAW produced by IDT 122 in response to a pulse received from arequesting unit by antenna 128 is reflected back toward the IDT 122 byIDT 136. A return RF signal is therefore transmitted to the requestingunit as discussed above in relation to FIG. 12, although the returnsignal generated by device 130 is not coded. The terminals of IDT 142are also either open or shorted, to thereby generate a second returnsignal to the requesting unit.

The middle IDT 138 is connected to a sensor 144, which may for examplebe a load impedance which changes according to a sensed characteristicor property such as moisture or temperature. A further reflected SAW,the magnitude and phase of which is dependent upon the impedance of thesensor 144, is generated by IDT 138 and results in a third RF returnsignal. The reflection characteristics and thus the magnitude and phaseof the RF return signal generated by the so-called reference IDTs 136and 142 are known, depending upon the open or short circuiting of theterminals. These reference return signals can be compared to the returnsignal generated by the IDT 138 to determine the state of sensor 144 andthereby the value of the measured characteristic or property.

The device 130′ shown in FIG. 15 is a fully differential realization ofthe device 130.

FIG. 16 shows a system into which passive SAW RF devices according tothe third embodiment of the invention could be implemented. A requestingunit 150, which may for example be a hand-held unit with a display orpart of a larger interrogation and tracking system, sends an RF pulse152 to a label, tag or the like generally indicated at 160. The tag 160includes a SAW device 120, 120′, 130 or 130′ and may be attached to orplaced on or inside an item or at a location where measurement by sensor144 is to be made. The return signals 154 generated by the SAW device intag 160, are received at the requesting device. For a device 120 or120′, which produces a coded return signal 154, the return signal isprocessed to determine tag information. For sensor applications in whichreference return signals and a sensor return signal are generated, thesignals are compared to determine sensor information. The tag or sensorinformation thus determined may for example be displayed to a user oroperator of the requesting device 150, forwarded from the requestingunit 150 to an information, tracking or billing system for furtherprocessing, or both.

It will be appreciated that the above description relates to thepreferred embodiment by way of example only. Many variations on theinvention will be obvious to those knowledgeable in the field, and suchobvious variations are within the scope of the invention as describedand claimed, whether or not expressly described.

The invention claimed is:
 1. A device comprising: a substrate; a firstinterdigital transducer (IDT) configured to receive a first electricalsignal and generate, from the first electrical signal, a surfaceacoustic wave that propagates along the substrate; and a second IDT thatis located on the substrate and has fingers that extend from twoopposite terminals and are configured based on a code, the two oppositeterminals being short circuited together so that the second IDT willreceive the surface acoustic wave from the first IDT and reflect, backto the first IDT, a surface acoustic wave that includes a code based ona configuration of the fingers; the first IDT being configured tooutput, from the reflected surface acoustic wave, a second electricalsignal that is based on the code.
 2. The device of claim 1 wherein thefirst IDT has two opposite terminals and fingers, the fingers extendingfrom the terminals and overlying the substrate.
 3. The device of claim 1wherein the first electrical signal is a single pulse.
 4. The device ofclaim 1 wherein the code is defined by relative positions of fingers ofthe second IDT.
 5. The device of claim 1 wherein the second IDT is athree-finger IDT in which, corresponding to each unit of the code, theIDT has one finger coupled to one of the terminals and two fingerscoupled to the other of the terminals.
 6. A device comprising: asubstrate; a first interdigital transducer (IDT) configured to receive afirst electrical signal and generate, from the first electrical signal,a surface acoustic wave that propagates along the substrate; and asecond IDT that is located on the substrate and has fingers that extendfrom two opposite terminals and are configured based on a code, the twoopposite terminals being open circuit terminated so that the second IDTwill receive the surface acoustic wave from the first IDT and reflect,back to the first IDT, a surface acoustic wave that includes a codebased on a configuration of the fingers; the first IDT being configuredto output, from the reflected surface acoustic wave, a second electricalsignal that is based on the code.
 7. The device of claim 6 wherein thefirst IDT has two opposite terminals and fingers, the fingers extendingfrom the terminals and overlying the substrate.
 8. The device of claim 6wherein the first electrical signal is a single pulse.
 9. The device ofclaim 6 wherein the code is defined by relative positions of fingers.10. The device of claim 6 wherein the second IDT is a three-finger IDTin which, corresponding to each unit of the code, the IDT has one fingercoupled to one of the terminals and two fingers coupled to the other ofthe terminals.
 11. A method comprising: receiving, by a firstinterdigital transducer (IDT), an electrical signal; generating, by thefirst IDT, from the received electrical signal, a first surface acousticwave; generating, by a second IDT, from the surface acoustic wave, afirst reflected surface acoustic wave, the second IDT having oppositeterminals that are open circuit terminated or closed circuited together;generating, by a third IDT, from the surface acoustic wave, a secondreflected surface acoustic wave, the third IDT having opposite terminalsthat are connected to a sensor that imparts, across the terminals, animpedance that is a function of a sensed property to cause the magnitudeor phase of the second reflected surface acoustic wave to be a functionof the property; outputting, by the first IDT, respectively from thefirst and second reflected surface acoustic waves, first and secondreturn electrical signals; and determining a value of the property basedon a comparison of said magnitude or phase of the first and secondreturn electrical signals to each other.
 12. The method of claim 11wherein the property is selected from temperature and moisture.
 13. Themethod of claim 11 wherein said magnitude or phase is magnitude.
 14. Themethod of claim 11 wherein said magnitude or phase is phase.
 15. Themethod of claim 11 wherein, in the receiving step, the receivedelectrical signal is received from an antenna, and, in the outputtingstep, the return electrical signals are output to an antenna.